Oxidation states of molybdenum in oxide films formed in sulphuric acid and sodium hydroxide

► Oxides are formed on molybdenum in sulphuric acid and sodium hydroxide solutions. ► Molybdenum IV and VI are identified by XPS, with MoO2 species dominating. ► Thicknesses of films are determined by ion beam analysis for a range of potentials. ► Films form at low efficiency due to loss of molybdenum species to electrolyte.